Evaluation of Methods for the Determination of Mercuric Chloride on Carbon R. L. MAUTE, R. H. BENSON, J. D. STROUD, NEIL HODGSON,' and D. R. BEASECKER' Monsanto Chemical Co., Texas City, Tex.
b Various methods for the determination of mercury on carbon impregnated with mercuric chloride and cerous chloride were evaluated. Both the amalgamation and the standard decomposition procedures yielded low and erratic results due to the nature of the sample. These low results are probably due to the formation of the known thermally stable complex of mercuric chloride-cerous chloride and losses during decomposition. To eliminate these errors, an isotope dilution technique using Hgz03 and a direct x-ray fluorescence method were developed. Both methods normally give comparable values. The x-ray procedure is fast and hence preferable for routine analyses; however, the equipment required for the isotope dilution technique is much less expensive. These two techniques appear to b e the only satisfactory methods for the accurate determination of mercury in this type of mixture.
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with a precision of 10.57, absolute was desired for the determination of high concentrations (0.5 to 18%) of mercuric chloride on carbon catalyst impregnatedwith mercuric chloride and cerous chloride. Gravimetric or titrimetric methods ( 7 ) are usually employed for the determination of mercury. In these methods, organic substances containing mercury are normally decomposed by the Carius method or by acid decomposition prior to the determination of mercury. Loss of mercury has been reported during the acid decomposition of organic compounds. In fact, mercuric chloride is known to be lost from aqueous solution when heated on the steam bath (5, 8 ) . 'Therefore, any method for mercury which involves a decomposition must take into account the volatility of mercury and its salts. Investigation of the sulfuric-nitric acid digestion technique followed by a standard titration procedure ( 7 ) gave correct values for knov n amounts of mcrcuric chloride mixed with carbon. However, attempts to determine knon n hiETHoD
Present address, hlonsanto Research Corp., Dayton Ohio. 02 Present address, Monsanto Chemical Co., St. Louis, Mo.
amounts of mercuric chloride impregnated on carbon containing cerous chloride invariably gave low results. Efforts to eliminate mercury losses by rigorous temperature control gave only limited success. I n addition the deconiposition of some samples was rather slow (1 t o 3 hours). The determination of mercury by amalgamation with gold was revised by Fahey (3) to determine total mercury in ores. Recently Brookes and Solomon ( 8 ) modified equipment to give good mercury recovery for inorganic and organic compounds in mixtures and seed dressings. Lacy (6) modified the amalgamation procedure for the determination of mercuric chloride on carbon. His apparatus minimized mercury losses and gave a standard deviation of *0.06% a t 3% mercuric chloride. The speed, simplicity, and accuracy of Lacy's amalgamation method led t o its investigation. The low results found here initiated the use of a n isotope dilution procedure. X-ray fluorescence spectrometry, the second method investigated, offers the advantage of a direct nondestructive analysis. Therefore, a large source of error, namely loss of mercury during decomposition, is eliminated. ISOTOPE DlLUTlO
METHOD
Reagents and Equipment. Hg203 KO3 solution, 1 pc. per ml. (specific activity 50 me. per gram of Hg), in 0.lX "03, from Oak Ridge National Laboratory. Scintillation counter (Nuclear-Chicago Model 2800 portable scaler) with shielded 2 X 2 inch sodium iodide crystal detector. Gold foil, 24 karat, '/4 inch by 2 inches, 30 mm X 75 mm X 0.0007 inch, Arthur H. Thomas Co. 50-ml. quartz round-bottomed flask with 19/38 female joint and a silica column (53/16inches long) nith a 19/38 male joint packed n-ith 41/2inches of gold foil. Procedure. Keigh accurately 0.5 t o 1.0 gram of the carbon catalvst into the flgsk. Keigh the silica "column to 0.1 mg. after packing with gold strips. Count both the contents of flask and column separately using the scintillation counter. They should be counted a t a fiwd distance of approximately 20 cm. from the detector crystal. ,4dd 1 ml. of Hg203solution to the flask and recount.
Swirl gently about 2 grams of a 50-50 mixture of iron filings (40 mesh) and calcium oxide (analytical reagent) with the carbon. Cover this mixture with a layer of iron filings and then a layer of calcium oxide. Place an asbestos shield around the silica column just beloy.the gold. It should fit tightly to m i m z e loss of mercury by volatilization. Heat the flask and column gently for 10 minutes, then increase the heat to a dull red color for 20 minutes. Allow to cool to room temperature and again count the contents of the flask. Wash out any calcium oxide in the bottom of the silica column with 3N HC1. Follow by washing with distilled water and acetone. Gently dry the column with a current of air. Place the column in a calcium sulfate desiccator for 10 minutes and reweigh. Recount the contents of the column. CALCULATIONS
C.p.5. of Hg203 on gold after heating C.P.S. of gold before heating = c.p.5. HgZo3gained C.p.5. of flask containing Hg203 - C.P.S. of flask without Hg203 = total C.P.S. Hg20Jused C.P.S. Hg203 gained on gold X 100 C.P.S. total Hg203 used yo Hg recovered Wt. of Hg on gold X 13,576 Sample weight x % Hg recovered yo HgCln in sample X-RAY FLUORESCENCE SPECTROMETRY
A General Electric XRD-5 x-ray spectrometer with a n argon - methane flow proportional Equipment.
counter, LiF analyzing crystal (2D = 4.0267) and 10-mil collimator was used. The tungsten target x-ray tube n-as operated at 50 kv. and 50 ma. Procedure. T h e second-order mercury Lai (1.241A.) is measured a t $6.09'. -4 background reading was taken a t 28 equal to $4'. The intensity of the mercury line was measured by counting for 100 seconds, four consecutive times, the average being taken. The background was then subtracted from this value. The calibration curve for mercuric chloride us. intensity gave a straight line. The samples were ground to -325 mesh. I n our laboratory, a Spes Mixer/hfill with a n alumina-ceramic vial was used for this purpose. Thp sample was then diluted with reagent grade anhydrous sodium borate, -325 VOL. 34, NO. 4, APRIL 1962
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mesh, in the ratio of one part of sample to ten parts of borax. This produces approximately equal absorption of all samples ( 4 ) . Homogeneity was ensured by first mixing in the mixer/mill, then by ball milling. Calibration of the x-ray spectrometer was achieved by synthetically prepared standards. Examination of the catalyst’s x-ray spectra also showed iron to be present. A matrix material was prepared using amorphous carbon (-325 mesh), and adding reagent grade ferric oxide and cerous chloride at levels approximating those of the catalyst materials to be analyzed, To different portions of this mixture were added varying amounts of mercuric chloride to give a series of mixtures ranging from 1 to 18% mercuric chloride. These were then diluted with borax in the same manner as the samples. l n e calibration was checked as .follows: A portion of the catalyst sample was heated in the absence of air, fhus removing the mercuric chloride from its matrix. Several heatings were necessary and after each, a qualitative check for mercury was made. Heating was continued until no detectable mercury remained. To this material was added a known weight of HgC12. The mixture was then diluted with sodium borate and run with the standards. -7
DISCUSSION
Isotope Dilution. Brookes and Solomon’s ( 2 ) method gave 99.1% recovery of mercury. They claimed only 1- to 2-mg. losses from 4 to 358 mg. of mercurous chloride. To eliminate losses of mercury during washing, they collected the distilled mercury on zinc wool and titrated it after acid dissolution. Lacy’s modified procedure (6) is accurate and faster than acid decomposition. In addition, his method can be performed by inexperienced personnel with good results. To determine the effectiveness of Lacy’s method, samples of mercuric chloride mixed with carbon were analyzed and low values (88 to 96%) were obtained. Carbon impregnated with mercuric chloride and cerous chloride gave even lower results, as shown in Table I. [These samples were prepared by a commercial manufacturer. The mercury content was stated by the manufacturer and found by x-ray fluorescence (XRF) to be correct. ] Apparently the carbon retained some mercury. The impression was later confirmed by the use of radioactive mercury. Two differences are evident between Lacy’s method and ours. We determined mercury present on carbon impregnated with cerous chloride whereas Lacy analyzed only mercury on carbon. Patents (1) state that cerous chloride and mercuric chloride form a thermally stable complex salt (CeC13.4HgC12) and 554
ANALYTICAL CHEMISTRY
that the mercuric chloride does not sublime during the reaction when it is used as a catalyst. The decreased volatility of mercury due to formation of this complex may cause low results. Secondly, Lacy’s procedure was varied slightly. The iron filings were 40 mesh (100 mesh specified) and to ensure intimate contact, the sample was mixed with the iron filings. This mixture was then covered with a layer of iron filings and calcium oxide. Brookes and Solomon specify only that fairly coarse iron filings be used. The procedures of Lacy and Brookes and Solomon are similar to our procedure used to reduce and distill the mercury. The differences are in the method of collecting the mercury (gold or zinc wool) and the method of determination (gravimetric or titrimetric), Since tracer techniques proved that low recoveries were obtained in the Lacy procedure, the procedure of Brookes and Solomon was not investigated. Since the recovery of mercury from our samples TVas variable, an isotope dilution technique should be applicable as it is uninfluenced by mercury losses. The only extra time involved is due to counting the labeled mercury. This technique was tried on samples of mercuric chloride impregnated on carbon and gave a precision (1 sigma limit) of h0.56 for six analyses a t a 10.72% level (see Table 11). The method was also found applicable for the determination of mercury in diphenyl mercury, the only organomercury compound tried. During the distillation procedure, the labeled Hgzosadded equilibrates and is reduced with the unlabeled mercury present. Hence, it is not necessary to impregnate tho sample with labeled mercury prior ‘.o analyses. Carbon dioxide, conveniently generated from Table IV.
Table 1. Recovery of HgClz Impregnated on Carbon Containing Cerous Chloride
Amalgamation and Gravimetric Method Known 12.0 17.5
% HgClz % Found Recovered 9.5,9.0,9.5 75-79 14.5, 15.0 83-86
Table II. Precision Study for Mercury b y Isotope Dilution Method on Carbon mpregnated with HgClz and CeCls
HgClz
Found
Deviation from Average
10.8 10.1 11.3 11.4 10.3 10.5 10.72 Av. 10.56 ( 1 )
+0.08
-0.62 +0.58 +0.68 -0.42 -0.22
Table 111. Precision and Accuracy of HgClz b y XRF on Carbon Impregnated with HgClz and CeCI3
,Known Found 12.0 12.1,11.9 12.0-12.5 12.3,12.3, 12.2 Unknown 3 .O, 3 . I (2% CeClr added)
sucrose, may be used to sweep over the mercury during distillation ( 2 ) . Our experience proved that this was not necessary. I n fact, its use frequently caused an error due to a residue deposited on the gold. In addition, no difference was found in results using calcium oxide or zinc oxide with iron to reduce the mercury. Reproducible results require fairly careful technique in the distillation. The time of heating should be uniform. Recovery of mercury on the gold is
Comparison o f Data from Different Methods for Mercury on Carbon Impregnated with HgClz and CeCh
Per Cent HgCI2 Found
Sample 1
2
3
4 5 6 12.0 HgC12 (Known) 7 8 9 10 11
12 13 14
Optical emission after acid decomposition” 0.67 3.5 8.6 7.8 10.7 10.8
...
Titration after acid decompositiona 0.67 2.29 5.83 7.39 8.25 9.16 5.4
...
... ..*
...
... ... ... ...
Nitric-sulfuric acid digestion.
...
...
X-ray
Isotope
fluorescence 3.1 3.6 9.1 9.4 11.3 12.0 12.1
dilution 4.2 2.9 8.3 9.1 10.9 12.4 11.7
7.8 9.1 10.4 12.0 17.1
7.4 8.9 10.8 11.7 17.0
11.3
14.7, 15.0
10.1 10.5
10.0 9.5
normally from 70 t o 90% (based on labeled mercury) depending on the degree of heating. The residue normally contains 2 to 6% labeled mercury. Repeated and prolonged heating failed t o remove this mercury residue completely, as measured by the labeled mercury. The volatile losses of mercury were normally 2 to 5% but OCcasionally were as high as 20%. This loss is again dependent upon the rate and time of heating. Both heating the column containing gold a t 105” C. for several minutes to ensure complete dryness and the use of an ether wash in lieu of acetone were tried. No losses were noted on drying; however, the ether rinse did give somewhat poorer precision. The best precision was obtained with a high specific activity radioactive mercury solution. The errors in the isotope dilution technique are larger a t the low concentration. These are primarily due t o errors in weighing from moisture, catalyst and reagent dust, sampling error, and counting errors. Table I1 gives a n example of the precision found by the isotope dilution method for one sample. X - Ray Emission Spectroscopy. Since carbon, a light absorber of x-rays, and mercury, a heavy absorber, vary in their relative amounts from specimen t o specimen, t h e total absorption also changes from specimen to specimen. Thus, direct measurement of samples and standards would not yield a linear working
curve. AS t h e dilution of the sample, with a relatively transparent material, is increased, the differences in composition of the original sample affect total absorption in a decreasing amount. When using sodium borate as the diluent, a tenfold dilution was necessary to obtain a linear working curve over the range of 1 to 18% mercuric chloride. Dilution with sodium borate offers another advantage. Since carbon, a major constituent of the catalyst samples, is a very light absorber of x-rays, it is inconvenient to measure enough sample so that infinite thickness is achieved,-Le., sufficient thickness so t h a t the intensity is independent of the thickness. Sodium borate, although relatively transparent, is a heavier absorber than carbon. Thus, the dilution with sodium borate permits the usage of a sample of a more convenient thickness. Incomplete mixing, uncorrected absorption effects, and ordinary instrumental fluctuations are perhaps the chief sources of error. Assuming mixing to be complete, a precision of =kO.l% and accuracy of =k0.2Y0absolute is estimated for this determination. Table I11 shows some results found. Analyses of a large number of samples of both known and unknown mercury content gave agreement satisfactory for our purposes. Some typical examples are given in Table IV. Part of the early differences found between X R F and isotope dilution values (for example NO. 13) were due to sampling errors,
since the carbon did not contain a uniform mercury content. Quartering the samples minimized this error. The comparison of mercury analyses by various methods is summarized in Table IV. The sulfuric-nitric acid digestion and emission procedures were developed and performed by an independent laboratory; the other analyses were made by company personnel. Both isotope dilution and x-ray emission spectrography have been used for routine analyses of mercury on carbon catalyst containing HgClz and CeC18. The x-ray method is preferred because of speed, accuracy, and precision, once calibrations have been prepared. LITERATURE CITED
(1) Boyd, T. (to Monsanto Chemical Co.), U. S. Patent 2,446,123-4 (July 27, 1948). (2) Brookes. H. E.. Solomon. L. E.. ‘ Analyst 84, 633 (1959). (3) Fahey, J. J., IND. ENG. CHEM., ANAL.ED.9, 477 (1937). (4) Gunn, E. L.. ANAL. CHEXI.29, 184 (1957).’ (5) Hillebrand, W. F., Lundell, G. E., “Applied Inorganic Analyses,” pp. 167 ff., Wiley, New York, 1929. (6) Lacy, J., Anal. Chim. Acta 20, 195 (1959). (7) Scott, W. W., “Standard Methods of Chemical Analysis,” 5th ed., pp. 574 ff., Val. I. W. H. Furman. ed.. Van Nostrahd, New York, 1958. (8) Zarkovskii, F. V., Aptechnoe Delo, 1952, No. 5, 35; C . A . 47,445f (1953). ’
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RECEIVEDfor review April 21, 1961. Accepted January 19, 1962.
Determination of Antimony by the Ring Oven Technique PHILIP W. WEST and ALBERT0 J. LLACER Coates Chemical I aboratories, I ouisiana Sfate University, Baton Rouge, I a.
b A method for the determination of antimony, suitable for use in air pollution studies, is presented utilizing solvent extraction of the Sbld- complex on the ring oven and subsequent development of color with phosphomolybdic acid. The limit of identification of the method is 0.08 pg. with no serious interferences for analyses of air samples. A modification of the Weisz ring oven to permit concentration of a sample and subsequent extraction of one component to the ring zone is also presented.
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that both antimony and its salts are poisonous. Fairhall and Hyslop ( I ) have reviewed the literature on the toxicology of antimony, and the analytical problems T IS WELL ESTABLISHED
associated with the determination of this metal have been discussed by Jacobs (3). The development of the ring oven technique now offers the possibility of providing relatively simple and rapid methods for separating, concentrating, and determining trace amounts of antimony collected during air pollution and industrial hygiene studies. The maximum allowable concentration for antimony has been given as 0.1 mg. per cubic meter of air. For convenience in sampling, a method sufficiently sensitive for samples of 1 cubic foot would be ideal. I n terms of absolute amounts of antimony, this requires a sensitivity of 2.8 pg. A sensitivity of 1 pg. per cubic foot as a minimum sensitivity would be more realistic so that an amount substantially less
than the maximum allowable concentration could be determined easily. Reasonable efficiency can be assumed for the collection of antimony dusts and fumes using paper and Millipore filters, impingers or electrostatic precipitators. Regardless of the method of sample collection, it can be assumed that any antimony isolated will be accompanied by possible interfering substances such as lead, iron, aluminum, copper, bismuth as well as certain nuisance dusts such as silica, silicates, and various carbonaceous materials. Therefore, the new method was designed to separate antimony from possible interferences, followed by a highly distinctive and sensitive discriminatory test utilizing a minimum of equipment and chemical operations. VOL. 34, NO. 4, APRIL 1962
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